PGE.sub.1, a substance having potent vasodilative and platelet aggregation inhibitory activities, is known to be a drug useful for the treatment of certain diseases such as chronic obstructive arterial diseases [thromboarteritis obliterans (TAO), arteriosclerosis obliterans (ASO)]. Nonetheless, because PGE.sub.1 is inactivated rapidly in the pulmonary circulation, continuous arterial therapy which calls for an extraordinary skill, intravenous drip injection, or massive administration has been attempted clinically but these therapeutic modalities unavoidably entail adverse reactions such as vascular pain, venotitis, and systemic hypotension.
Recently, for overcoming the above disadvantage of PGE.sub.1 and exploiting the outstanding efficacy of the drug, many explorations have been made for formulating PGE.sub.1 into fat emulsions (Japanese Kokai Tokkyo Koho S58-222014, Kokai Tokkyo Koho H4-69340, Kokai Tokkyo Koho H4-338333, Kokai Tokkyo Koho H4-338334, Kokai Tokkyo Koho H4-338335, etc.). Actually, the PGE.sub.1 fat emulsion-described in Japanese Kokai Tokkyo Koho S58-222014 is commercially available today.
The above commercial PGE.sub.1 fat emulsion is a useful PGE.sub.1 preparation, for PGE.sub.1 entrapped in emulsion vehicles is inactivated in the lungs only to a limited extent and this advantage coupled with the vascular lesion-targeting effect of the emulsion due to the characteristic distribution of its fine vesicles in the body produces a potent and lasting vasodilative effect at a low dose, not to speak of a reduced risk for local ADRs, for example mitigated irritancy. It is reported that this fat emulsion contains approximately 20% of decomposition products of PGE.sub.1 even immediately following its manufacture and that the PGE.sub.1 content declines at a rate of about 2.5% per month even when stored at 5.degree. C. [Kazuo Mizuguchi et al., The Clinical Report, 26, 1647-1653 (1992)], thus being not impeccable in quality and stability.
As a means for insuring a sufficient stability of PGE.sub.1 in a fat emulsion, lyophilization of such a PGE.sub.1 -containing fat emulsion was contemplated and actually much study has been undertaken for implementing a lyophilized fat emulsion (PCT WO92/07552, Japanese Kokai Tokkyo Koho H5-43450, Japanese Kokai Tokkyo Koho H6-157294, etc.). However, none of the efforts have borne fruit.
Meanwhile, each drug has a definite pH range in which it is stable and barring an extraordinary circumstance, pharmaceutical products are conventionally formulated within the pH range in which the active substance is stable for insuring a reasonable shelf-life. In the case of PGE.sub.1, it is known to be stable in the weakly acidic region (Pharmaceutical Research, Vol. 5, No. 8, 482-487 (1988); Pharmaceutical Research, Vol. 6, No. 3, 210-215 (1989)), and the above-mentioned fat emulsion available on the market has also been adjusted to a weakly acidic region (pH 4.5-6.0).